In order to reduce the percent defective, semiconductor devices are manufactured in a clean room where dust and the like are present in an extremely small amount. It is required that the air inside such clean room be free from not only dust, but also ammonia. In cases where the air inside the clean room contains ammonia even in a low concentration of several ppb, the accuracy of ultra-fine process-patterning is deteriorated and the percent defective is increased. Therefore, the ammonia concentration of the air inside the clean room must be monitored at all times.
At present, the ammonia concentration of the air inside a clean room is determined by passing the air in the clean room through about 100 mL of a collection solution at a prescribed flow rate using a pump to collect ammonia in the air into the collection solution, which is then transferred to an analysis center where ammonia in the collection solution is quantified.
However, this method requires about 3 days from the start of the measurement to acquisition of analytical result, including the time required for transferring the sample to an analysis center; therefore, prompt action cannot be taken even when the ammonia concentration becomes high. In addition, the ammonia detection limit of this method is about 50 ppb, which is not sufficient.
It is demanded to develop a method of measuring the ammonia concentration in the air, which does not require transferring a sample to an analysis center and is capable of performing a measurement on site in a clean room in a short period of time, preferably in about 20 minutes, and whose measurement sensitivity is in the order of 1 ppb.
The present inventors developed a method of measuring the ammonia concentration in the air using a microchannel chip (Non-patent Document 1). In this method, an air containing ammonia, which is a test sample, and a collection solution are introduced into a microchannel where a gas-liquid two-phase flow is formed and ammonia is extracted from the gas phase into the liquid phase. The two-phase flow is allowed to pass through the microchannel having a hole of 2 mm in diameter on the top and the gas phase is discharged to convert the two-phase flow to a liquid-phase flow. Then, to this liquid-phase flow, a coloring solution and oxidizing solution for ammonia analysis are introduced to color the liquid-phase flow, and the color is measured by a thermal lens microscope (TLM) to determine the ammonia concentration.
According to this method, the ammonia concentration in the air can be measured on site in a period of about 10 minutes in the order of 1 ppb; therefore, the above-described demand can be satisfied.
However, in this method, the measurement reproducibility is low and the coefficient of variation (CV) of the measured values is 20%. Therefore, it is demanded to develop a measurement method having a higher reproducibility.